This is a calcium matte base (as opposed to the magnesia matte G2934). The clay is Plainsman M390. 5% Zircopax was added on the left (normally 10% or more is needed to get full opacity, the partially opaque effect highlight contours well). 5% tin oxide was added to the one on the right (tin is a more effective, albeit expensive opacifier in oxidation). The PLC6DS firing schedule was used.
G1214Z - Cone 6 Silky Matte
This glaze was born as a demonstration of how to use chemistry to convert a glossy cone 6 glaze into a matte.
Random material mixes that melt well overwhelmingly want to be glossy, creating a matte glaze that is also functional is not an easy task.